Traditionally, communication of digital intelligence requires that a bit stream, representing the intelligence to be transmitted, be employed as a modulating signal to modulate a carrier, thus producing an analog waveform carrying the same intelligence. After transmission, a digital demodulator recovers the digital intelligence by processing the received signal. Typically, the digital demodulator produces an analog signal which carries the intelligence that has been transmitted, and further demodulation recovers the intelligence.
One of the corrupting influences present in the analog signal is DC offset; efficient and effective demodulation requires the reduction of this DC offset. DC offset is introduced at many points in the communication channel, typically, such offset is produced in a mixer included in the demodulator. One well-known technique for eliminating DC offset is by employing a capacitor in the signal path, which capacitor is effective to block DC levels. This technique, referred to as AC coupling, cannot be employed where the demodulator is intended to be pattern insensitive. Regardless of the particular type of modulation employed, pattern insensitivity requires that the digital demodulator respond correctly to a repeating sequence of binary signals of one type or the other. Such a repeating sequence is manifested, in the demodulator, as a DC level, and AC coupling the analog signal will erroneously remove this DC level.
Digital demodulators typically include either hard-decision demodulation or soft-decision demodulation. In hard-decision demodulation, information is extracted from the analog signal by noting the polarity at specified points in time. Soft-decision demodulation decodes or converts the analog signal into a multi-bit digital output which is representative, not only of the polarity (which is the sole bit of information employed in hard-decision demodulation) but the amplitude as well. Those skilled in the art will understand that soft-decision demodulation is more capable than hard-decision demodulation, but carries a requirement of greater circuit complexity.